Effect of Carbon Content on Impact Properties of 9%Cr Ferrite Heat-Resistant Steel

9%Cr ferritic heat-resistant steel with carbon mass fractions of 0.01%, 0.05%, and 0.10% was prepared by vacuum melting, forging and heat treatment. The effect of carbon content on the microstructure and impact properties of the steel was studied. The results show that the test steel with different carbon content was composed of tempered martensite lath and precipitates, and precipitates included M₂₃C₆ phase distributed at grain boundaries and martensite lath boundaries and MX phase distributed in martensite lath. With the increase of carbon content, the average grain size of the test steel first decreased and then remained basically unchanged, the width of martensite lath first decreased and then increased, and the content and size of M₂₃C₆ phase increased; the upper shelf energy of the test steel first increased and then decreased, and the ductile brittle transition temperature first decreased and then increased. The influence of grain size on the ductile brittle transition temperature of the test steel was significant, while the size and content of M₂₃C₆ carbides had a relatively pronounced effect on the upper shelf energy. The test steel with a carbon mass fraction of 0.05% had the best impact properties with the ductile brittle transition temperature of −90.0 ℃ and the upper shelf energy of 341.0 J.